The present invention relates to heavy duty industrial gas turbines used in electrical power generation, and in particular, to a method for operating a multi-stage axial flow compressor component of such turbines during power grid under-frequency events, and a strategy for transitioning between nominal and under-frequency operation schedules.
Large increases in the electrical power consumptive demand placed upon an electrical power distribution grid will tend to reduce the electrical operational frequency of the grid, causing an xe2x80x9cunder-frequencyxe2x80x9d event. For example, a heavy or sudden electrical demand may cause a particular power distribution grid having a nominal operational frequency of 50 Hz to momentarily operate at 49 Hz. In conventional electrical power generation systems that utilize one or more heavy-duty industrial gas turbine for supplying electrical power to the grid, the physical speed of each turbine supplying power to the grid is synchronized to the electrical frequency of the grid. Unfortunately, as the physical speed of a gas turbine decreases, other things being equal, its power output correspondingly decreases. Consequently, during an under-frequency event, a gas turbine will tend to output a lower power. In the past, a common practice in response to a power grid under-frequency event (occurrence) is to increase the firing temperature of the gas turbine to produce more power in an effort to maintain a predetermined level of output power. Unfortunately, such over-firing of the gas turbine results in drastically reducing the operational life expectancy of various hot gas path components within the turbine.
Although some heavy-duty gas turbines conventionally used for power generation have been known to incorporate variable inlet guide vanes, the use of variable stator vane in addition to variable inlet guide vanes has been relatively uncommon prior to the introduction of General Electric""s H-Series Gas Turbines. Such variable stator vanes provide the ability to adjust airflow incidence angle (i.e., the difference between the air angle and the mean line angle at the compressor blade leading edge) in the front stages of the compressor so that an acceptable compressor surge-free operation margin may be maintained. Typically, maintaining surge-free operation is a vital critic-to-quality (CTQ) operational criterion of the compressor component for these types of gas turbines.
The inventors of the present invention recognized that the variable stator vanes could be used to modify the airflow volume consumed by the compressor component and, thus, modulate the output power produced by the gas turbine.
In one aspect, the present invention overcomes problems associated with over-firing of gas turbines equipped with variable stator vanes (blades) to compensate for power output during under-frequency events by utilizing the variable stator vanes to increase the amount of airflow consumed by the compressor component in a predefined manner so to preclude and/or minimize a decrease in the level of output power generated rig a grid under-frequency event. In another aspect, the present invention overcomes urge problems associated with increasing the power output of a gas turbine equipped with variable stator vanes by maintaining operation within a safe surge margin during the occurrence of a power grid under-frequency event. In a further aspect, the present invention overcomes potential operational problems that may occur as a result of switching between nominal operating conditions and power grid xe2x80x9cunder-frequencyxe2x80x9d operational conditions.
Varying the angle of the inlet and stator vanes of the compressor component alters the overall airflow volume consumed by the compressor and affects the resultant turbine output power produced. The method of the present invention relates to gas turbine systems wherein one or more of the front variable stator vanes (VSV) of the compressor component are ganged together by means of a common actuation mechanism. Operational schedules for varying angular positions of the ganged stator vanes with respect to corrected physical compressor speed are defined for both normal and xe2x80x9cunder-frequencyxe2x80x9d power grid operating auditions to provide optimum compressor efficiency without violating minimum safe compressor surge margin criteria. A compressor operational method and strategy is provided for controlling the angular position of the ganged compressor vanes in a manner that ensures smooth transitions between nominal and under-frequency (or vice versa) operational schedules during and/or subsequent to the occurrence of power grid under frequency events.